The invention relates to an arrangement for raster scanning the ocular fundus utilizing a laser light source.
Arrangements of the kind referred to above are known as scanning laser ophthalmoscopes (SLO or LSO). Scanning ophthalmoscopes are utilized for performing fluorescence angiography.
The largest possible viewing field is observed during the inflow phase of the fluorescence material. In the late phase, interesting areas, such as the capillary system, are observed with higher spatial resolution. For this purpose, it is known to change the imaging scale of the apparatus used. The number of image points per image is usually constant. From this, it becomes necessary to reduce the image field to increase the spatial resolution.
The following solutions are known:
(1) To increase the resolution, the apparatus includes, for example, a changeable optical element, such as a movable mirror as disclosed in European patent publication 0,495,469. The actuation of this optical element leads to the condition that the scanned field changes its size. The aspect ratio of the field does not change and both fields have the same center point. A simultaneous shift of the center point is not possible. The shift of the center point is necessary in order to obtain a high resolution image in the peripheral region and is done manually by the operator. For this purpose, the entire scanning system must be pivoted about the pupil of the patient.
(2) The apparatus scans over the entire region unchanged but only when the laser beam is disposed in the selected area is the detected signal utilized to build up the image (it is possible that also the laser is switched off during scanning of the area which is not selected in order to minimize the burden to the patient). This requires the digitalization rate to be changeable. The disadvantage here is that the recording time per pixel reduces when the image section is smaller and therefore the number of photons received per pixel. This leads to a reduction of the signal/noise ratio and therefore to a reduction of the image quality.
In U.S. Pat. No. 5,396,302, the imaging scale (and therefore the scan angle in the horizontal direction) is changed via two exchangeable mirrors. A scan angle change in the vertical direction is controlled via the control of a galvanometer scanner.
In European patent publication 0,279,589, eye movements are compensated by tracking utilizing two galvanometer scanners.
In European patent publication 0,615,721, acoustic-optical deflectors as horizontal scanners as well as resonance scanners are described in an arrangement of three or more scanners. Resonance scanners afford the advantage compared to galvanometer scanners that they have a higher frequency of oscillation and comprise a pivot mirror on a torsion rod in a housing. The mirror is electrically driven to oscillate at the natural frequency about its center point. Resonance scanners afford the advantage relative to polygon scanners that the scan angle is changeable via the electrical drive control.
It is an object of the invention to make different areas of the ocular fundus accessible to the viewer during the scanning operation without deteriorating the image quality.
The laser scanning ophthalmoscope of the invention is for viewing the ocular fundus of the eye of a patient. The laser scanning ophthalmoscope includes: a laser source generating a laser beam; a first scanner for imparting an oscillatory deflection to the laser beam in a first direction; a second scanner for imparting an oscillatory deflection to the laser beam in a second direction and for transmitting the oscillating laser beam toward the ocular fundus whereby at least a portion of the light of the laser beam is reflected from the ocular fundus; detector means for detecting the light reflected from the ocular fundus; a display for displaying an image defined by the light reflected from the ocular fundus; control means connected to the display for selecting a sectional image of the image and the sectional image corresponding to a subregion of the ocular fundus; a drive motor for pivotally moving at least one of the first and second scanners; a drive unit for controlling the drive motor; and, an interface for connecting the control means to the drive unit.
The invention makes it possible, during the scanning operation, to scan a smaller area or subregion of the entire scannable region at a high line frequency. This subregion can be at any desired location in the entire scannable region and the scanning of the subregion takes place with the same pixel frequency and with the same image buildup time as scanning the entire scannable region.
Accordingly,it is made advantageously possible to change the subregion to be scanned in size and position during the investigation with a scanning ophthalmoscope.
The center point of the new subregion to be scanned must not be in the center point of the previous subregion to be scanned. It is therefore no longer necessary for the operator to move the apparatus. Furthermore, the subregion of interest remains continuously under observation during the zooming operation.
Furthermore, the invention permits the subregion to be scanned to wander over the entire region to be scanned without it being necessary for the operator to move the apparatus.
The laser scan ophthalmoscope according to the invention continuously defines an image of the ocular fundus on a PC monitor. If a subregion of the image is of special interest, then this subregion can be selected by key or lever combinations or by pulling a window with the mouse of the PC. A control unit (PC) detects the coordinates of this subregion and emits control signals to the drive or control of the horizontal and vertical scanners as well as to a pivot unit. The scanners thereupon change their amplitudes and their oscillating zero points so that only the previously selected subregion is scanned. A galvanometer scanner is usually utilized as a slow vertical scanner. For such a scanner, the amplitude of the scan and the center of the scan can be varied without additional complexity by varying the amplitude or the offset of the control voltage.
Rapid mechanical horizontal scanners do not operate pursuant to this principle and therefore do not afford the possibility of changing the subregion to be scanned while maintaining a constant pixel frequency. This problem is solved by the electronic/mechanical solution according to the invention.
Several advantageous embodiments are provided for changing the subregion to be scanned at least in the direction of the rapid horizontal scan.
The scanning ophthalmoscope includes a motorized control which can pivot the entire scanning system (both scanners including housing) about the pupil of the patient. The motorized pivot arrangement is controlled by a PC.
The resonance scanner is securely held in a rotational bearing. In this rotational bearing, the resonance scanner can be rotated by a motor about its oscillatory axis. A rotation about this axis shifts the zero point of oscillation of the scanner and therefore shifts the center of the subregion to be scanned in the horizontal direction.
As will be explained below, this motorized control is driven by a control unit.
The scan system includes an adjustable mirror or scanner whose shift or scan direction is parallel to the direction of the rapid horizontal scanner. This mirror or scanner is preferably disposed between the rapid horizontal scanner and the slow vertical scanner. The center of the scan shifts in the direction of the rapid horizontal deflection because of a rotation of this additional mirror or scanner.
In view of the above, it is possible to do the following according to the invention:
(a) select and scan a desired subregion of the total region to be scanned;
(b) move the subregion to be scanned over the entire scannable region via a key control or by the operator;
(c) move the subregion to be scanned on a predetermined path over the entire scannable region and record images during this movement; and,
(d) place the images one next to the other via image processing in order to obtain a high-resolution total image.